1. Field of the Invention
The present invention generally relates to IDcell allocation. More particularly, the present invention relates to an apparatus and method for allocating IDcells in a broadband wireless communication system.
2. Description of the Related Art
Institute of Electrical and Electronics Engineers (IEEE) 802.16e communication systems are basically cellular systems. Since the communications systems use a frequency reuse factor of 1, the same frequency can be used in adjacent cells. Therefore, a Mobile Station (MS) must be able to distinguish its serving sector from neighbor sectors among sectors using the same frequency. For this purpose, each sector transmits its specific Pseudo Noise (PN) code in the first symbol of every frame, referred to as a preamble.
The IEEE 802.16e standard defines 114 preamble PN codes in total, indexed from 0 to 113. Each preamble PN code carries an IDcell and a segment number. Hence, the MS can acquire the code index, IDcell, and segment number of the sector by interpreting the preamble PN code. The IDcell is a number from 0 to 31 and the segment number is 0, 1 or 2. This means that every code cannot have a unique (IDcell, segment number) combination. Only codes 0 to 95 have unique (IDcell, segment number) combinations and codes 96 to 113 reuse (IDcell, segment number) combinations.
IDcell serves a variety of purposes, especially for subcarrier randomization, which significantly affects system performance. During modulation, each sector performs subcarrier randomization using a Pseudo-Random Bit Sequence (PRBS). The initialization vector of a PRBS generator is designated 11 bits, b0 . . . b10, such that
<Downlink>
b0 . . . b4=five Least Significant Bits (LSBs) of IDcell in the first Partial Usage of SubChannels (PUSC) zone or downlink PermBase.
b5 . . . b6: PRBS_ID (segment number+1 in the first PUSC zone).
b7 . . . b10: 0b1111 (all 1s).
<Uplink>
b0 . . . b4=five LSBs of IDcell.
b5 . . . b6: 0b11 (all 1s).
b7 . . . b10: four LSBs of Frame Number.
For the downlink, the initialization vector is generated using an IDcell and a segment number in the first PUSC zone. For 32 IDcells and 3 segment numbers, 96 initialization vectors are created. For the uplink, the initialization vector is generated using an IDcell and a frame number. The number of initialization vectors varies with how a frame number is used. For instance, if frame numbers are not synchronized between sectors, up to 512 initialization vectors are generated, whereas if frame numbers are synchronized between sectors, 32 initialization vectors are generated. That is, the number of PRBSs that can be generated for the uplink is limited to 32. Then, the probability that two or more adjacent sectors perform the same randomization becomes three times higher in the uplink than in the downlink. To avoid the same subcarrier randomization in adjacent sectors, the same IDcell is not allocated to the sectors, in the case of frame number synchronization, and the same (IDcell, segment) combination is not allocated to the sectors, in the case of frame number asynchronization.
The same subcarrier randomization between adjacent sectors leads to the same constellation mapping in the adjacent vectors. As the MS cannot identify a desired signal from interference, the MS faces a serious problem during demodulation, especially as the MS suffers from a great degradation of pilot estimation performance. Accordingly, it is preferred that adjacent sectors use different IDcells and there exists a need for a method for allocating IDcells so as to minimize the cases of using the same IDcell in sectors.